"Advice" is a mass noun.  Amend text accordingly.

functions.texi: (Advising Functions, Core Advising Primitives)
(Advising Named Functions, Advice combinators)
(Porting old advice): Replace, e.g., "an advice" with "advice".
"Advice" is a mass noun.  Amend text accordingly.

cl.texi (Obsolete Macros): Replace "an advice" with "advice".
Merge branch 'master' of git.sv.gnu.org:/srv/git/emacs